Fluid flow is an important consideration in a number of laboratory processes. Microfluidic technologies, in particular, have recently captured widespread attention in the fields of biological assays, clinical diagnostics, and synthetic and analytical chemistry. These technologies represent a significant advancement, particularly for those applications such as proteomics and genomics, in which expensive or rare fluids are employed. Typically, microfluidic systems miniaturize and automate a number of laboratory processes that are then integrated on a chip. Each chip may contain a network of microscopic channels through which fluids and chemicals are transported in order to perform experiments. Thus, microfluidic devices are particularly suited for the analysis of minute sample quantities.
Microfluidic devices are typically produced employing the same microfabrication methods that are used to make microchips in the computer industry, enabling the creation of intricate, minute patterns of interconnected channels. Once a pattern is created, microchip manufacturing methods are employed to recreate the channel design in a substrate formed from a rigid material such as silicon, quartz, glass, or plastic. This process allows the development of highly precise channels with dimensions that can be varied in their width and depth. Once the pattern is produced in the substrate, a rigid cover plate is affixed over the substrate so as to form conduits in combination with the channels. Because microfluidic devices may be constructed using simple manufacturing techniques, they are generally inexpensive to produce.
Microfluidic devices do, however, present certain technical challenges that need to be overcome. For example, fluid flow characteristics within the small flow channels of a microfluidic device may differ from fluid behavior in larger devices, since surface effects tend to predominate, and regions of bulk flow become proportionately smaller. Consequently, several techniques have been developed in order to achieve fluid flow control in microfluidic devices.
Fluid movement in microfluidic devices sometimes involves electrokinetic flow, which is generated by electrodes in reservoirs at each end of a channel that are activated when an external power source applies a voltage across the electrodes. Under these conditions, fluids of the appropriate type will move by electroosmosis, a process that precisely and controllably generates linear flow rates within the channel, typically about a millimeter per second. Electrophoresis, another electrokinetic phenomenon, may also occur in the channels. This involves the movement of charged molecules or particles in an electric field. Electrophoresis is often used in conventional laboratories for analyzing molecules according to their chemical structures. Electrophoresis can be used to move molecules in solution, or to separate molecules with subtle chemical variations. Electrophoresis and electroosmosis generally occur in channels simultaneously, as described, for example, in U.S. Pat. No. 5,876,675 to Kennedy.
In addition, or in the alternative, pressure can be used to move fluid in the channels. For example, U.S. Pat. No. 6,117,396 to Demers describes a device for delivering defined volumes of a liquid. The device employs one or more sources of gas to pressurize metering capillaries containing liquid therein and to expel liquid therefrom. Generally, on the microfluidic scale, small amounts of pressure may produce predictable and reproducible fluid flow through microfluidic fluid-transporting features. However, the size of fluid-transporting features plays an important role in fluid flow. U.S. Pat. No. 6,268,219 to Mc Bride et al. describes a fluid distribution system that may be used to evenly distribute fluid to a plurality of channels that branch successively from a main channel by controlling the size of apertures that serve to couple the main channel with the branching channels.
A number of patents describe various mechanical valve technologies that in theory may be employed in microfluidic devices. U.S. Pat. No. 4,869,282 to Sittler et al., for example, describes a micromachined valve that employs a control force to deflect a polyimide film diaphragm. The polyimide film diagraph is sealed to a micromachined silicon layer having flow channels on a surface thereof. U.S. Pat. No. 5,368,704 to Madou et al. describes a micromachined valve that can be opened and closed electrochemically. The micromachined valve operates by employing an electrolytic film material, which may be repeatedly dissolved and redeposited in and from a compatible electrolyte to open and close the valve. Suitable materials include, for example, metals, such as silver or copper, or electroactive polymers, such as polypyrrole. Successful integration of mechanical valve structures into microfluidic devices, however, remains an elusive goal.
In addition, there are a number of other drawbacks and limitations in microfluidic device construction. For example, the stiffness of the materials used for microfluidic device formation necessitates high actuation forces, which in turn may result in large and complex designs. To overcome limitations associated with ordinary microfluidic devices formed from rigid materials, microfabricated elastomeric valve and pump systems have been proposed in WO01/01025. Similar valves and pumps are also described in Unger et al. (2000), “Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography,” Science 288:113-116. These publications describe soft lithography as an alternative to silicon-based micromachining as a means by which to form microfluidic devices. Through soft lithography, microfluidic structures created entirely from an elastomer may be constructed containing on/off valves, switching valves, and pumps. The structures comprise an elastomeric block having a microfabricated recess formed therein. Actuation of a portion of the elastomeric block results in the deflection of that portion into the recess. Thus, the microfluidic valves described in this publication are subject to disadvantages, such as always remaining open in an unactuated state. Furthermore, the valves typically require mechanical or pressure-based actuation and are not easily adapted for electrical control.
Electroactive polymers have been described in Peirine et al. (2000), “High-Speed Electrically Actuated Elastomers with Strain Greater than 100%,” Science 287:836-839, as well as in a number of PCT publications. (See WO01/58973, WO01/59852, WO01/06575, and WO01106579.) Such electroactive polymers represent a low-cost, high-performance actuator material capable of converting electrical energy into mechanical energy, and are of particular interest because they can be tailored to suit specific purposes. For example, the electroactive polymers described in these publications have been employed to form transducers, such as in the conversion of electrical energy into mechanical energy (and vice versa). By applying an electric field to at least two electrodes that are in contact with the electroactive polymer, the polymer may be deflected due to linear elastic strains in excess of about 100 percent. Such deflections may be exploited for use in fluid flow control devices, particularly in microfluidic or small devices.
Thus, there exists a need in the field of fluid flow control, particularly in microfluidics, to employ elastic materials as alternatives to already known fluid flow control technologies. Electroactive polymers, in particular, make it possible for electrically controllable valve and pumps to provide mechanical fluid control.